Next time someone asks you who you think you are, just give them the facts. You're a mammalian amniotic tetrapodal sarcopterygiian osteichthyan gnathostomal vertebrate cranial chordate. Yeah, it's a mouthful. And in order to understand what it means, you're going to have to understand the most complex group of animals on earth, and what it takes to get from this to this.

(Intro)

The phylum Chordata accounts for all 52,000 species of vertebrates on Earth and several thousand species of invertebrates. Together they range from tiny, brainless filter feeders all the way up to Scarlett Johansson. Now, you know by now that when we talk about classifying animals, we're really talking about their shared ancestry, each new branch on this tree marking an important new evolutionary milestone. And just like with tissue layers and segmentation in simpler animals, there are traits we can look for to track the evolution of chordates. By the time all of those traits appear in one organism, we'll have arrived at the most complex class within the most complex phylum: the mammals.

Chordate Synapomorphies
(1:04) But first let's start with the fundamentals. We've talked before about synapomorphic traits: traits that set a group of animals apart from its ancestors and from other groups that came from the same ancestors. Chordates share four synapomorphies that make us who we are. Each of them is present at some point in every chordate's life cycle.

Cephalochordata
(1:20) How about a volunteer to demonstrate these traits? Ah, I see that the lancelets are raising their... mouthparts. The lancelets, also known as cephalochordata, literally "head-cords," are one of the three sub-phyla of chordates. And unlike almost all other chordates, these tiny, brainless, invertebrate filter feeders retain all four of these characteristics for their entire lives. You probably already know where most of these traits are going to appear, since the phylum is named after it: the spinal cord, or at least something that resembles a spinal chord. First there's the notochord, a structure made of cartilage that runs between an animal's digestive tube and its nerve cord. In most vertebrates, a skeleton develops around the notochord and allows the muscles to attach. In humans, the notochord is reduced to the disks of cartilage that we have between our vertebrae. Second, we have the nerve cord itself, called the dorsal hollow nerve cord a tube made of nerve fibers that develops into the central nervous system. This is what makes chordates different from other animal phyla, which have solid, ventral nerve cords, meaning they run along the front or stomach side. Third, all chordates have pharyngeal slits. In invertebrates like the lancelet here, they function as filters for feeding. In fish and other aquatic animals, they're gill slits, and in land-dwelling vertebrates like us they disappear before we're born, but that tissue develops into areas around our jaws, ears, and other structures in the head and neck. And finally, we can't forget our fourth synapomorphy, the post-anal tail, which is exactly what it sounds like. It helps propel aquatic animals through the water, it makes our dog look happy when she wags it, and in humans it shrinks during embryonic development into what is known as the coccyx, or tailbone. It's right here. And trust me, when it comes to tail placement, post-anal is the way to go. These four traits all began to appear during the Cambrian explosion more than 500 million years ago, and today they're shared by members of all three chordate sub-phyla even if the animals in those sub-phyla look pretty much nothing like each other.

Urochordata
(3:16) For instance, our new friends here in cephalochordatan are the oldest living sub-phylum, but you can't forget the other invertebrate group of chordates, the urochordata, literally "tail-cords." There are over 2,000 species here, including sea squirts. And if you're confused about why this ended up in a phylum with us, it's because they have tadpole-like larvae with all four chordate characteristics. The adults, which actually have a highly developed internal structure, with a heart and other organs, retain the pharyngeal slits but all the other chordate features disappear or reform into other structures.

Vertebrata
(3:49) The third and last and most complex sub-phylum, is the Vertebrata, and has the most species in it because its members have a hard backbone, which has allowed for an explosion in diversity, from tiny minnows to the great blue whale. You can see how fantastic this diversity really is when you break down Vertebrata into its many, many classes from slimy, sea-snakey things to us warm and fuzzy mammals. And as these classes become more complex, you can identify the traits they each developed that gave them an evolutionary edge over the ones that came before. For example, how's this for an awesome trait: a brain! Vertebrates with a head that contains sensory organs and a brain are called craniates. They also always have a heart with at least two chambers.

Myxini
(4:30) Since this is science, you're going to have to know that there's going to be an exception for every rule that you're going to have to remember and the exception in this case is the Myxini, or hagfish, the only vertebrate class that has no vertebrae but is classified with us because it has a skull. This snake-like creature swims by using segmented muscles to exert force against its notochord. Whatever, hagfish.

Petromyzontida
(4:51) Closely-related to it is the class petromyzontida, otherwise known as lampreys, the oldest living lineage of vertebrates. Now these have a backbone, made of cartilage, and maybe even more important, a more complex nervous system. With the advent of the backbone we see vertebrates getting larger, developing more complex skeletons, and becoming more effective at catching food and avoiding predators. But do you notice anything missing? Lampreys and other early vertebrates are agnathans, literally "no-jaws." And if you want to be able to chew food, it really helps to have a jaw and teeth. Most scientists think that the jaw evolved from structures that supported the first two pharyngeal slits near the mouth. And teeth? Well, the current theory is that they evolved from sharp scales on the face!

Chondrichthyes
(5:32) Gnathostomes, or "jaw-mouths", arrived on the scene 470 million years ago, and one of the oldest and most successful groups of gnathostomes that have survived to the present day are the class chondrichthyes, the "cartilage fish". You know them as the sharks and skates and rays, and as their name says, their skeleton is made up mostly of cartilage, but they show the beginnings of a calcified skeleton. Chondrichthyans haven't changed much over the past 300 million years or so, and their success stems from the paired fins that allow for efficient swimming, and those jaws for biting off delicious hunks of flesh.

Osteichthyes
(6:03) If we're going to eventually get to the mammals, we need bones, and we find those with the evolution of fish. Meet Osteichthyes, which technically means "the bony fish" unlike cartilaginous fish, members of this group have a mineralized endoskeleton. Now, Osteichthyes is sometimes considered a superclass, because it includes a whole slew of diverse classes that descended from it. There's actually some controversy among taxonomists about what to call it, but the main thing to know is that the majority of all vertebrates fall under Osteichthyes, and that includes you. It's broken up into two main groups, which themselves include a bunch of classes. The first is Actinopterygii, or ray-finned fishes, and with 27,000 species, pretty much every fish you've ever heard of is in this group. Ray-finned fishes evolved in fresh water, spread out into the oceans, and some eventually came back to fresh water. In the second group, things start to get really strange and interesting. These are the lobe-finned fishes, or sarcopterygii, a name derived from bones surrounded by muscle found in their pectoral and pelvic fins. And that sounds like something that could be used for walking! Lobe-fins include the coelacanths, which consist of one living species, lungfish, which gulp air into their lungs; and tetrapods, which have adapted to land with four limbs. So this is weird, right? Even though land animals clearly are not fish, since tetrapods evolved from bony fish, they are filed under this group. These taxonomists, man. I want to party with them sometime. But first:

Bio-lography
(7:29) Imagine that you're a fisherman off the coast of South Africa in the western Indian Ocean about 75 years ago. Just put that in your brain and hold on to it. And you've just pulled up a fish that no one has ever seen. Not only that, you've caught a fish that was thought to have become extinct 75 million years ago. This is exactly what happened in 1938, when Captain Hendrick Goosen hauled up a coelacanth, and it has mystified scientists ever since. A second population has since been identified near Indonesia in 1999, but the deep-sea creatures remain extremely rare. The coelacanth fascinates scientists because of its paired lobe fins. They extend from the body like legs and move in an alternating pattern. In other words, they move more like a horse than like a fish. And in fact those paired fins are supported by the very same bones that we have in our arms and legs. The coelacanth also has a hinged joint in the skull so it can widen its mouth to eat large prey, as well as thick scales that don't exist on any living fish. It's not good eating, but why would you want to eat what's essentially a living fossil? 

Alright, now we're talking about tetrapods, which of course means "four feet," and getting those four feet onto land was really awesome for those early creatures because that meant that they could escape the increasingly brutal and predatory world of the ocean. Tetrapods gradually replaced their fins with limbs, and developed entirely new body parts that were never seen before, like necks, with the help of additional vertebra, that separated the body from the head. 

Amphibia
(9:02) The first tetrapods are today found in the class Amphibia, which were the first creatures to develop a three-chambered heart. There are more than 6,000 known species of amphibians like frogs and salamanders, most of which begin life as tadpoles in water, and then later develop legs, lungs and a digestive system, and often migrate to land for adulthood. But amphibians lay eggs that don't have shells, so they dehydrate quickly, so they have to be laid in water. So this leads us to our next evolutionary milestone for chordates: the amniotic egg. Amniotes are tetrapods that have eggs adapted for life on land a group that includes reptiles, birds and mammals. The amniotic egg was crucial for the success of land-dwellers, allowing embryos to develop in their own "private pond" of the amniotic sac, often surrounded by a hard shell in the case of reptiles and birds.

Reptilia
(9:47) The class Reptilia represents the earliest amniotes. Like amphibians, they have a 3-chambered heart, but they're totally terrestrial. And here's where we find our dinosaurs and our snakes and turtles and lizards. You often hear reptiles described as "cold-blooded." This does not mean that their blood is cold. They're actually ectothermic, which means that they absorb external heat as their main source of body heat. Hence the lizard that likes to lay in the sun all day. The oldest group of reptiles, the archosaurs, mostly disappeared when most of the dinosaurs died out 65 million years ago. But two lineages of archosaurs survived. One includes the modern reptiles crocodiles and alligators, and the other is a type of dinosaur that we now call birds, the class Aves. There are big, obvious differences between these two surviving archosaurs: one is designed for, like, eating and fighting big animals, while the other is designed for flying around and being graceful and stuff. The not-so-obvious but equally important difference is that birds are endotherms, which means that they can crank up their metabolism to regulate their body temperature. Making all that heat requires a big furnace, which is provided thanks to the evolution of a four-chambered heart. 

Mammalia
(10:47) There's only one other group of animals that developed this trait, independently of birds, by the way, and it allowed them to spread through the planet. I'm talking, of course, about the class Mammalia, otherwise known as amniotes that have hair, three special ear bones and mammary glands. And most mammals have evolved to dispense with the hard egg shell altogether the embryo avoiding predation and other environmental dangers by developing inside the mother's body. In this class of chordates you'll find me, Dame Judi Dench, your dog, your cat, Shamu the orca, African elephants, the South American pudu, and 5,300 other known species of mammals. It all began with a simple ancestor more than 500 million years ago in this crazy chordate phylum, but we've finally made it! And now you know exactly who you are.

Thanks for watching this episode of Crash Course Biology. We hope that it was helpful and that you feel like a smarter person. There's review stuff next to me that you can click on to go to those parts of the video that you maybe want to watch a little bit more of; to reinforce it in your brain. Thanks to everybody who helped put this together. And if you have any questions for us you can get in touch with us on Facebook or Twitter or, of course, down in the comments below. Goodbye.